Yucca Mountain site must make use of geological safety net, say IU and Los Alamos scientists

A proposed federal repository near Yucca Mountain, Nev., for the long-term storage of 70,000 metric tons of high-level radioactive waste must take advantage of the mountain's geological properties, according to a new study by scientists at Indiana University Bloomington and Los Alamos National Laboratory.

The report, published in the November-December issue of American Mineralogist and largely funded by the U.S. Department of Energy's Yucca Mountain Site Characterization Office, provides the most detailed three-dimensional picture to date of the minerals most likely to impact long-term waste storage. Conclusions from the paper will likely be used during licensing discussions before the U.S. Nuclear Regulatory Commission next year.

"The repository must not place undue reliance on any one portion of the storage system, such as the man-made engineered portion," said David Bish, Haydn Murray Chair in Applied Clay Mineralogy at IUB and the report's lead author. "The long-term storage of high-level radioactive waste will depend on geological and engineered systems that are intertwined in a complex way."

Bish and his Los Alamos National Laboratory colleagues also confirmed that Yucca Mountain's rocks are rich in zeolites -- soft, clay-like minerals with powerful absorption properties. The mountain's zeolite deposits, which include extensive layers of clinoptilolite and mordenite, have long been considered a major asset that supports any proposed storage facility. This is because zeolites are known to readily absorb a number of positively charged ions, such as radioactive cesium, barium and strontium.

Zeolites also possess some of the properties of sponges, absorbing and releasing large amounts of water. This water can, in turn, absorb much of the heat produced by emplaced radioactive materials, providing an outlet for that energy. Clinoptilolite and mordenite do not, however, effectively absorb several radioactive ions that are negatively charged or are very large. Bish said that this doesn't necessarily make Yucca Mountain a less desirable waste storage site.

"These zeolites are still one of the most potent natural means of retarding the movement of radioactive ions through rock," said Bish, who left Los Alamos to join the IUB Department of Geological Sciences earlier this year.

Bish also said that understanding the complex geological and mineralogical features of Yucca Mountain is vital to modeling the long-term performance of any storage facility that's built there. And although the sponge-like properties of Yucca Mountain minerals are desirable for a radioactive waste repository, Bish said a facility that successfully prevents waste migration will require a combination of man-made safeguards and the mountain's natural features.

The researchers took more than 2,000 samples from 17 cored holes across Yucca Mountain, at depths ranging from 20 to 1,800 meters below the surface. They performed X-ray diffraction studies of each sample. Bish and his colleagues found zeolites are widespread at many depths, but perhaps most importantly, are abundant at a depth considered ideal for waste storage -- 300 meters below the mountain surface and 150 meters above the water table. A waste containment area would be built above this zeolitic safety net.

"I'd like to know more about how introduction of a repository into the mountain will change the geology, mineralogy and hydrology, all of which will affect the ability of the mountain to contain the waste," Bish said. "We also need to know more about how water flows through the repository horizon, a zone of rock into which the waste would be placed."

Yucca Mountain is located about 100 miles northwest of Las Vegas, near the DOE's Nevada Test Site. It was initially suggested as a possible high-level radioactive waste repository in the late 1970s. Congress approved the site for waste storage in July 2002. DOE representatives are expected to approach the U.S. Nuclear Regulatory Commission in 2004 to acquire three federal licenses: one for facility construction, another permitting the storage of high-level radioactive waste and a third for sealing the repository.

"Because the work presented in our paper provides the most comprehensive, three-dimensional mineralogical picture of Yucca Mountain, we believe the paper will probably be used in licensing deliberations," Bish said.

If all goes according to proponents' plans, the Yucca Mountain site could begin receiving radioactive waste as early as 2010. Many obstacles exist, however, that will likely delay the transportation of waste. Many states object to the transportation of radioactive waste across their borders. Also, the state of Nevada contests scientists' claim that Yucca Mountain is geologically satisfactory for the purposes of high-level radioactive waste storage.

Some funding for the study came from other DOE sources. Los Alamos National Laboratory scientists David Vaniman, Steve Chipera and William Carey were coauthors of the report.